A 4 COMPARTMENT MODEL OF PHYTOPLANKTON CHLOROPLASTS FOR THE CARBON CONCENTRATING MECHANISM

The common observation that the affinity of most phytoplankton for carbon dioxide is much
higher than that of RuBisCO has led to proposing carbon concentrating mechanisms (CCMs).
While several algae can utilise bicarbonate, which is converted to carbon dioxide via
carbonic anhydrase (CA), others have been shown to rely solely on carbon dioxide uptake.
Ideas put forward to explain algal CCMs include lowering external pH and extracellular CA,
active uptake od carbon dioxide and bicarbonate, as well as processes within the chloroplast.
Recently, a role of low thylakoid lumen pH in combination with intrathylakoidal CA was
suggested; the presence of CA in the thylakoid lumen and the chloroplast envelope was
demonstrated. Here we present a detailed mathematical model based on chloroplast fine
structure, membrane permeability, and the distribution of pH and CA. Fluxes and reactions
within and between the envelope, girdle lamella, stroma, and thylakoids are incorporated.
The model generates near saturating carbon dioxide concentration for RuBisCO in the stroma
and is consistent with current understanding of carbon-isotope fractionation.